Week 2 Heat Treatment and the Effect of Welding

7/30/2019 Week 2 Heat Treatment and the Effect of Welding

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Heat Treatment and the Effect

of Welding

Week 2


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Heat Treatment of Steels

The basis of heat treatment is that FCC iron

can dissolve all carbon in steel (up to 2%

C), while BCC iron can dissolve practicallynone (


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Steel Phase Diagram


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Contd

On cooling the carbon will attempt to precipitateout of solution as Cementite

By controlling the mode of cooling thedistribution of Cementite & hence the mechanicalproperties can be controlled

Steels are heated slowly to the Austenite region (+30 to 50 C) to ensure it is fully Austenitic & thatthe grains are as small as possible

Final properties depend on the mode of cooling


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Cooling

Annealingusually on cast & hot workedsteels with coarse grain structures to obtain

grain refinement, stiffness & ductility

Particularly necessary on components

requiring additional work

Involves cooling slowly in the furnace orpacked in sand


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Contd

Hardeningquenching into oil, water or brinefrom the soak temperature fast enough to prevent

the formation of PearliteNew phase known as Martensite (supersaturated

solution of carbon in ferrite) very hard & as a

result the steels become very brittle

With water quenching the steel becomes too brittle

for use becomes necessary to tempersteel


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Contd

Tempering re-heating to the sub critical range(approx 650 C), where stresses set up on

quenching are relieved, so reducing the brittlenessSteel becomes tougher at the expense of hardness

Quenching & tempering are principally applied to

high carbon steels, where high hardness is

required or to alloy steels to achieve high strength


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Welding

Extensively used for joining materials together

Very complex geometries can be effectivelywelded

Produces cleaner lines and reduces painting costs

Cheaper, simpler & lighter than rivets or bolts

The material is heated locally to its melting

temperatureAdditional metal may be introduced and the jointis then allowed to cool naturally


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Contd

Allows greater freedom for design

Allows for continuous beams & girders

Easy & quick alterations

Additions can easily be made


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Methods Available

Arc welding

Gas welding

Friction weldingSpot welding

Soldering

Brazing

Electron beam

Laser

Diffusion bonding


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Ideal Requirements of Welding

A.) Complete continuity should be

maintained between parts to be joined

Joint should be indistinguishable from theparent metal

Practically the above is not always possible,

although satisfactory weld performance canbe achieved in most cases


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Contd

B.) The joining material should have

properties that are similar to the parent

metalCareful selection of welding rods etc. is

therefore essential


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Heat Affected Zone

Weld is basically a rapidly formed casting

surrounded by a heat affected zone (HAZ)

A temperature gradient is set up in thematerial during welding

Temperature gradient ranges from the

melting point at the point of fusion toambient temperature at some distance from

the weld


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Contd

High temperature followed by fairly rapid

cooling causes changes in the metallurgy of

the metal and the joint quality can beaffected by:

a.) Structure & quality of the weld metal

b.) Structure & properties of the part ofthe metal in the heat affected zone


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Rate of Cooling After Welding

The slower the rate of cooling, the closer

the structure to equilibrium

Cooling occurs mainly by conduction in theparent metal, depending upon the thermal

mass (thickness & size of parent material)

The greater the thermal mass, the faster therate of cooling


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Arc Welding

The main method employed for structuralsteelwork is arc welding

Principles Electrode or filler wire melts due topassage of welding current through the filler wire,Arc (plasma) & back to the power source via theearthed component

Typically arc temperature is 5000 to 30000K

The melt is transferred across the arc severalmechanisms droplets, spray etc.


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Welding Process

Basically require:

1.) Heat source to effect fusion

2.) Satisfactory metallurgical properties

3.) An efficient process


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Processes used

Manual metal arc

Automatic welding using continuous coated

electrodes

Submerged arc welding

Carbon dioxide shielded metal arc (MIG)

Electrostatic welding

Stud welding


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Typical Welds

Butt Weld

Full penetration

Partial penetration


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Contd

Fillet Weld

l1t

l2

t = throat

l1 = vertical leg

l2 = horizontal leg


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Defects

Residual stresses

Distortions

Undercut

Incomplete penetration

Porosity

Slag inclusion


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Weld Metal Solidification

Cracking

Weld metal solidification cracking hot

cracking longitudinal in a fillet weld

blue appearance (oxidised surface) due tomaterial composition and/or weld restrain &

bead shape


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Heat Affected Zone (HAZ)

Cracking

Heat Affected Zone (HAZ) Cracking heataffected zone due to weld adjacent to bead

affected by heat input & cooling cycle

depends on composition but cooling ratecan affect microstructure hardening more brittle carbide formation

Susceptibility also affected by hydrogen inthe weld metal introduced from the weldrod which is consumable


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Carbon Equivalent

Metal arc welding of carbon & carbon

manganese steels need to be checked by

reference to BS EN 1011 2: 2001 guidance on carbon equivalents suggests

suitable preheat levels to reduce cooling

rate for various thicknesses & limits onhydrogen levels sometimes need post heat

(heat treatment)


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Empirical Formula

15/)(5/)(6/ CuNiVMoCrMnCCE

C & Mn have a significant effect

Cr, Mo, Ni, Cu have little effect

Limited usually to

CE value


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HAZ Cracking

HAZ

HAZ crackWeld bead


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Lamellar Tearing

Associated with non-metallic manganese

sulphides & silicates when rolled material is

extended as planer type inclusions (likewrought iron)

Welds run parallel to inclusions & cracks

are induced through contractile stressingacross thickness of the plate


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Lamellar Tear Diagram

Lamellar tear

Inclusions

thin planer

types


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BS 4360 Steel (grade 50C)

Typical ladle analysis:

C = 0.21%

Mn = 1.50%

Cr = 0.025%

Mo = 0.015%

Ni = 0.04%

Cu = 0.04%

Determine the carbon

equivalent & comment on

weldability

C b E i l f BS 3460


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Carbon Equivalent of BS 3460

Steel

15/)(5/)(6/ CuNiVMoCrMnCCE

%473.0

005.008.025.021.0

15/)04.004.0(5/)015.0025.0(6/5.121.0

CE


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Comments on Weldability

Few problems are encountered at values


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Contd

If, of the elements in this formula, only

carbon and manganese are stated on the mill

sheet for carbon and carbon manganesesteels, then 0,03 should be added to the

calculated value to allow for residual

elements.

Where steels of different carbon equivalent

or grade are being joined, the higher carbon

equivalent value should be used


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Weld Decay in Stainless Steel

WeldHeat Affected Zone

Grain boundaries (scale ofgrains grossly exaggerated)

Region depleted of

chromium & no longer

stainless is attacked

preferentially by corrosion


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Welding & Structural Steels

Designed to be weldable

No serious loss of performance in the weld

or the HAZStructural engineers make allowance for

HAZ in the design process (typically a

20N/mm2 reduction in the yield strength isapplied)


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Electric Arc Welding


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Electric Arc Welding Equipment


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Use of Electric Arc Welding


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Metal Arc Inert Gas Shielded


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MIG Equipment


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Use of MIG Equipment


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Butt Weld


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Slag Inclusion


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X-Ray Testing

Documents

Week 2 Heat Treatment and the Effect of Welding